There is a pressing need for the development of mucosal vaccines against HIV-1. For this reason, over the last decade, our group has pursued attenuated Salmonella typhi as a mucosal delivery system for HIV-1 antigens. This work has led to the first Phase I clinical trial of attenuated Salmonella typhi expressing a truncated gp120 protein (tgp120) in the bacterial periplasm. Using that work as a backdrop, we are now positioned to evaluate the ability of attenuated Salmonella typhi to deliver a DNA vaccine expressing gp120 of the HIV-1Ba-l isolate (gp120Ba-l) in human volunteers after oral inoculation . Accordingly, the principal goal of this project is to obtain safety data in human volunteers for a gp120Ba- L DNA vaccine delivered by attenuated Salmonella typhi. There is also a pressing need for broader and more effective immunogens than gp120 to be put in any delivery vehicle used as a vaccine against HIV-1. To this end, preclinical studies in Projects 1 and 2 explore the safety and immunogenicity of a new immunogen, a single chain scgp120Ba-L-CD4 chimera that uses gp120 of the HIV-1Ba-L isolate (scgp Ba-L-CD4). This immunogen is predicted to elicit broadly neutralizing antibodies against HIV-1 If warranted by the preclinical data from those projects, Project 3 is also designed to obtain safety data in human volunteers on scgp120 Ba- L-CD4 delivered as a DNA vaccine by attenuated Salmonella typhi and as a soluble subunit protein given intramuscularly. There are 3 specific aims: 1) to determine the safety of a gp120Ba-L DNA vaccine delivered by attenuated Salmonella typhi in human volunteers (Study 1). To our knowledge, this will be the first Phase I study in volunteers to a bacterial vector to deliver a DNA vaccine. A such, it will provide the initial safety data required for further exploration of this vaccine strategy in Aim 2 or Aim 3, one of which will be carried out depending on the preclinical data obtained in Projects 1 and 2; 2) to determine the safety of a scgp120 Ba- L-CD4 chimera in human volunteers (Study 2). If warranted by preclinical data (Projects 1 and 2), both a scgp120Ba-L-CD4 DNA vaccine delivered by attenuated Salmonella typhi and a purified scgp120Ba-L-CD4 subunit protein will be evaluated for safety in normal healthy volunteers (Study 2). The long-term goal of this study is to develop the components to use scgp120Ba-L-CD4 in a prime-boost strategy; 3) to determine the safety of a gp120Ba-L DNA vaccine delivered by attenuated Salmonella typhi using multiple inoculations in human volunteers (Study 3). If the preclinical studies from Projects 1 and 2o do not support carrying out Aim 2 above, an expanded Phase I study will be carried using attenuated Salmonella typhi to deliver the gp120Ba- L DNA vaccines used in Aim 1. This study (Study 3) will determine the safety of multiple inoculations of this vaccine on immunity to gp120Ba- L.